Digitizer tablets are well known in the art. See for example the review in the January 1989 issue of BYTE magazine, pages 162-174. In certain applications, back-lighting of the working surface is highly desirable. Such applications include accurate tracing of drawings, or in CAD/CAM uses. Moreover, such applications often require the use of a larger size tablet, for example, with a 36.times.48 inch working surface. To accomplish back-lighting, it is known to mount fluorescent lamps under the working surface. This creates certain problems, because the working surface is the top surface of a sandwich structure which typically includes the electrode array and an apertured shielding electrode below the electrode array. Parts of these elements are typically light opaque and, being interposed between the light source and the working surface, cause observable grid patterns on the working surface. These patterns, unconnected with the drawings being processed, can interfere with the user's task and are thus undesirable.
In addition, because of the large dimensions of the tablet top and its weight, support members are necessary between the tablet top and the base in the space occupied by the light sources. Moreover, in use, the user leans on the working surface and presses his pointing device on the surface, additional reasons why support for the tablet top must be provided at multiple positions in the light source space. However, the presence of these support members tend to cause dark spots where they contact the tablet top. This also interferes with the proper tablet use.